Chargeable and dischargeable secondary batteries such as lithium secondary batteries have come to be widely used in recent years. Typically, such lithium secondary batteries are provided with a positive electrode and a negative electrode containing, respectively, a positive electrode active material and a negative electrode active material that are capable of storing and releasing lithium ions, a separator that is interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte that impregnates the positive electrode, the negative electrode and the separator. To produce the lithium secondary battery, the positive electrode, the negative electrode and the separator are assembled, and are impregnated with the nonaqueous electrolyte, followed by charging.
As is known, external foreign metals such as iron (Fe) or the like can be inevitably mixed into the secondary battery during production of the secondary battery. It has also been observed that when such a foreign metal is present in the vicinity of the positive electrode, the foreign metal dissolves in the nonaqueous electrolyte during charging of the battery (for instance, during initial conditioning), and goes on precipitating locally at sites opposing the negative electrode, as a result of which the separator may be pierced and short-circuits may occur. Accordingly, various methods have been proposed (hereafter also referred to simply as abatement treatment methods of foreign metals) that are carried out, after construction of a secondary battery and before initial conditioning, with a view to preventing short-circuits in the secondary battery caused by foreign metals in the secondary battery (for instance, Patent Literature 1 to 4).
For instance, Patent Literature 1 discloses a method that involves providing a standing time, ranging from one hour to 48 hours, after charging to a battery capacity of 0.01% to 0.1% during initial charging.
Further, Patent Literature 2 discloses the feature of applying an electrical shock of charge and discharge, or a combination of charge and discharge, to a battery, such that the shock is applied under a condition whereby the positive electrode potential is 4.0 V or higher and the negative electrode potential one minute after the battery is left to stand, subsequently to application of the shock, is 2.0 V or higher, with respect to Li.
In these methods, the foreign metal diffuses homogeneously in the electrolyte, without precipitating on the negative electrode.
Apart from abatement treatment methods of foreign metal such as those described above, self-discharge inspection is ordinarily performed that involves determining the occurrence or absence of internal short circuits by measuring the amount of voltage drop (self-discharge amount), in an unloaded state, of the secondary battery after initial conditioning. Self-discharge inspection involves checking the occurrence or absence of micro-short-circuits caused by foreign metal precipitation. However, inspection lasting 5 or more days, for instance about 10 days, was required to check the occurrence or absence of micro-short-circuits caused by precipitation of high-resistance iron.